Workaround for graphviz crash bug

[kno10]

This seemingly trivial change appears to avoid the graphviz crash bug.
But I don't know if this sometimes will produce less pretty plots.

[kno10]

Hi @dspinellis,
Would you have a look whether the port specification is needed at all (is the landingPort ever anything else than the entire class?), or whether we can add this workaround (using the center of the port, rather than the bounding box)?
But I noticed that my trimmed down test no longer crashes here. I will have to try the full version again.

[dspinellis]

The landing port can point to specific elements; see e.g. this example.

[kno10]

Does not appear to be necessary anymore for me. Closing.